<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <meta http-equiv="X-UA-Compatible" content="ie=edge">
  <title>Document</title>
  <style>
    .father{
      width:300px;
      height:300px;
      border:1px solid #f00;
    }
    .son1,.son2 {
      width:100px;
      height:100px;
    }
    .son1 {
      margin-bottom: 50px;
      background-color: purple;
    }
    .over {
      overflow: hidden;
    }
    .son2{
      background-color: pink;
      margin-top: 100px;
    }
  </style>
</head>
<body>
  <div class="father">
    <!-- 用over将son1、son2隔开，那么son1 就和son2不是同一个BFC -->
    <div class="over">
      <div class="son1"></div>
    </div>
    <div class="son2"></div>
  </div>
</body>
</html>